The present invention relates to a technique to improve developing properties of different color components in an image forming apparatus (electrophotographic apparatus), such as a color laser printer by which multicolor images can be provided, and more particularly relates to a technique to improve developing properties in a highlight (low density) region.
With reference to FIGS. 8 to 11, an outline of the process from development to print in a color laser printer will be described as follows. In FIG. 8, the surface of a photoreceptor drum 82 is electrically charged by a charging unit 81. Then, the surface of the photoreceptor drum 82 is raster-scanned by laser beams modulated by an image signal corresponding to a yellow toner image, so that a latent image of yellow is formed. After that, the latent image is developed by a developing unit 83 with yellow toner. In this case, the image signal is supplied by a color scanner not shown. (In the case of a copier, the color scanner is integrally assembled to the apparatus.) Alternatively the image signal is supplied by a computer in the form of data on one screen for each toner color. Then, the same operation as that described above is repeated for each of magenta, cyan and black toners, so that four color toner images can be formed on the surface of the photoreceptor drum 82. After that, the four color toner images are concurrently transferred onto a transfer sheet by a transfer pole 84. Then the transfer sheet is separated from the photoreceptor drum 82 surface by the action of a separation pole 85, and the transferred toner image is fixed by a fixing unit 86. In this way, a full color image can be provided.
In this case, the image signal is a multiple-valued signal (8 bits=256 gradations). Therefore, dot formation must be conducted on the photoreceptor drum 82 in a multiple-valued manner. For that reason, a time modulation system of laser beams has been conventionally known, in which pulse width modulation is performed.
The time modulation system of laser beams will be described as follows:
As shown in FIG. 9, an inputted multiple-valued digital image signal is converted into an analog image signal by a D/A converter 87. The converted signal is compared by a comparator 89 with a reference signal (in this case, a triangular wave) generated by a reference signal generation circuit 88. Thus a time modulation pulse is provided. A laser unit is controlled to be turned on and off by the provided time modulation pulse. As a result of the foregoing, a beam scanning operation of limited size corresponding to the lighting time is carried out as illustrated in FIGS. 10A through 10E, and a multiple-valued latent image is formed from dots of different sizes. When the latent image is developed by toner, multiple-valued density recording is performed.
In this connection, the conventional latent image which is composed of dots of each color is controlled in a printing process so that the dots are completely superimposed. Therefore, as shown in FIG. 11, concerning the color in a highlight (low density) region, latent images formed through time modulation conducted in a short period of time are superimposed on a toner image. Therefore, it is difficult to accurately control the deposition of toner to be superimposed. As a result, unevenness tends to occur on a color image in the process of color reproduction in the important highlight region.